Voyager 1 is on the edge of the sun's heliosphere and close to entering… (NASA / JPL-Caltech )
Thirty-five years after NASA's Voyager 1 was launched, the spacecraft is on the edge of the solar system and verging on entering interstellar space, the agency said Friday. The craft is now 11.1 billion miles from Earth, a distance that means radio signals from the craft require 16 hours and 38 minutes to reach the antennas of NASA's Deep Space Network. "It is very exciting. We are approaching the solar system's frontier," said Ed Stone, Voyager project scientist at Caltech.
Three instruments onboard are providing key data about the craft's passage into interstellar space. One measures the flux of cosmic rays from space. The sun's heliosphere diverts many of the cosmic rays that normally would enter the solar system. Voyager 1 had been measuring an increase in galactic cosmic rays, suggesting it is nearing the edge of the heliosphere. From January 2009 to January 2012, there was an overall increase of about 25% in the number of such cosmic rays the craft was encountering. Beginning on May 7, the flux began increasing by about 5% per week, Stone said.
The second important instrument measures the flux of energetic particles generated inside the heliosphere. That flux has been declining gradually. Researchers expect the flux to drop precipitously when the craft exits the heliosphere.
A third instrument measures the direction of the magnetic field lines around the spacecraft. While Voyager 1 is still within the heliosphere, those field lines run east-west. When it passes into interstellar space, they will run north-south. But analysis of that effect will take several weeks, Stone said.
Voyagers 1 and 2 were launched in 1977 to explore the outer solar system. Voyager 2 is now 9.1 billion miles from the sun.